The present invention concerns a mass spectrometer for analyzing compounds in a solution and a combined device comprising a separation means in a liquid phase such as a liquid chromatograph and a mass spectrometer.
At present, importance is posed on a highly sensitive detection method of chemicals contained in solutions in the analytical science field. For example, with an increasing interest on ecological problems, regulations on chemicals contained in city water have become stringent year by year. Therefore, kinds of substances as objects for regulation and monitoring have been increased and the standard value for each of the substances has tended to be lowered. Since a mass spectrometer (hereinafter simply referred to as MS) has high sensitivity and excellent ability of identifying substances, it is effective for the analysis of chemicals in solutions. In particular, for the analysis of mixtures, it has been expected that a combined device comprising a separation means in a liquid phase such as a liquid chromatograph (hereinafter simply referred to as LC) or a capillary electrophoresis (hereinafter simply referred to as CE), and MS.
FIG. 5 shows a schematic configuration of a conventional ion trap mass spectrometer (refer to Analytical Chemistry, 62 , 1284 (1990)). In the constitution of the prior art device, the polarity of a voltage applied to each of electrodes is selected depending on the polarity of ions to be analyzed. For the sake of simplicity, explanation will be made to a case of analyzing positive ions. A sample solution is introduced by way of a liquid feed pump 1 and a pipeline 2 to a metal tube 3. When a positive voltage at several kilovolts relative to an electrode 4 is applied to the metal tube 3 by a power supply 50, the sample solution is subjected to electrospray from the end of the metal tube 3. The liquid droplets formed by spraying contain a great amount of positive ions concerned with substances as an object for analysis. Since the liquid droplets are dried in the course of flying in atmospheric air, gaseous ions are formed. The thus formed gaseous ions enter through a first aperture 5, a differential pumping region 7 evacuated by a vacuum system 6a and a second aperture 8 into a vacuum region 20 evacuated by a vacuum system 6b. A voltage referred to as a drift voltage is applied between and electrode 4 disposed with the first aperture 5 and an electrode 9 disposed with the second aperture 8. The application of the drift voltage provides an effect of accelerating the ions and colliding them against residual gas molecules thereby eliminating solvent molecules attached to the ions and an effect of improving the ratio of the ions passing through the aperture 8 (transmission efficiency). The electrode 9 disposed with the second aperture 8 is grounded to the earth. For focusing the ions, electrostatic lenses 10a and 10b are disposed to the differential pumping region 7 and the vacuum region 20 respectively. The ion trap mass spectrometer comprises two endcaps 12a and 12b and a ring electrode 13. A high frequency voltage is applied to the ring electrode 13, to form an ion confining potential within an inner space 21 of the mass spectrometer 11. The inner space 21 of the mass spectrometer is at a pressure of about 1031 3 Torr by the introduction of a helium gas referred to as a collision gas. Ions injected from an ion entrance opening 14 disposed to the endcap 12a collide against the helium gas molecules to lose their energy and confined by the confining potential in the mass spectrometer. After accumulating the ions in this way for a predetermined period of time in the space 21, the amplitude of the high frequency voltage applied to the ring electrode 13 is changed thereby making the trajectory of the ions unstable in the space 21 and the accumulated ions are ejected from the ion exit opening 15. Since the condition for making the ion trajectory unstable is different depending on the value obtained by dividing the mass (m) of the ion with the static charge (z) (m/z value), information on the m/z value of the ion can be obtained based on the amplitude value of the high frequency voltage applied on the ring electrode 13. Ions ejected from the exit opening 15 are detected by a detector 16, the detected signals are sent to a data processing device (not illustrated) and subjected to data processing. In FIG. 5, are shown electrodes 101 and 102 constituting the electrostatic lenses 10a, and electrodes 103, 104, and 105 constituting the electrostatic lens 10b. 
The conventional ion trap mass spectrometer described above involves a problem that the ion detection sensitivity lowers if the drift voltage is increased. Since ions of polar compounds such as peptides have a number of solvent molecules such as water attached thereto, a high drift voltage is necessary for effectively removing such attached solvent molecules. Accordingly, it has been impossible to analyze polar compounds such as peptides at high sensitivity by the conventional ion trap mass spectrometer.
The reason is considered as below. In the ion trap mass spectrometer, the energy of ions injected to the mass spectrometer is important due to the necessity of accumulating the ions in the mass spectrometer. The injected ions lose their energy upon collision with the collision gas in the mass spectrometer and are accumulated in the mass spectrometer. If the injected energy of the ions is excessively high, their energy cannot be taken away completely by the collision against the collision gas but the ions pass through the mass spectrometer. Since it has been considered so far that the energy of the ions injected to the mass spectrometer 11 is given by a potential difference between the electrode 9 having the second aperture 8 and the endcap 12a having the ion entrance opening 14, both electrode 9 and the endcap 12a are put at a ground potential in the conventional ion trap mass spectrometer to eliminate the potential difference between both of them, thereby intending to obtain a state in which the energy of the ions injected to the mass spectrometer 11 is reduced to substantially zero. However, it is, actually considered that ions are accelerated to a certain extent of energy by the drift voltage at an instance passing through the second aperture 8. Since the pressure in the differential pumping region 7 is relatively high and the ions frequently collide against the residual gas molecules, it is difficult to exactly recognize the energy of the ions upon passing through the second aperture 8. However, it is considered, a possibility that the energy of ions injected to the mass spectrometer 11 depends on the drift voltage. Accordingly, it is considered that if the drift voltage is increased, the injected energy of the ions is increased thereby lowering the ion confining efficiency and, as a result, the detection sensitivity of the ions is lowered.
As has been described above, the mass spectrometer having the differential pumping region 7 requires a high drift voltage as already described for analyzing the polar compounds at a high sensitivity. However, in the conventional device constitution, if the drift voltage is made higher, the ion detection sensitivity is rather lowered and, after all, to lower the analyzing sensitivity.
It is accordingly an object of the present invention to provide an ion trap mass spectrometer in which the ion detection sensitivity is not lowered even if a high drift voltage is used and which is suitable to highly sensitive analysis for polar compounds.
For attaining the foregoing object, in accordance with the present invention, a decelerating electric field forming means is disposed between the electrode having the second aperture and the endcap having the ion entrance opening. Actually, by providing a potential difference of a polarity to decelerate ions between the electrode having the second aperture and the endcap having the ion entrance opening, ions accelerated to a high energy by a drift voltage can be injected after being decelerated to a low energy into the mass spectrometer. Further, by controlling the intensity of the decelerating electric field such that the injected energy of the ions to the mass spectrometer can be maintained constant even when the drift voltage is changed, a good ion detection sensitivity can be obtained.